1. Field of the Invention
The present invention generally relates to systems for controlling sheetmaking machines and, more particularly, to systems for the control of calendering systems that employ multiple actuators.
2. Related Art
In the sheetmaking art, it is common practice to make on-line measurements of sheet properties. The purpose of on-line measurements is to enable prompt control of sheetmaking processes and, thus, to increase sheet uniformity and to reduce the quantity of substandard sheet material that is produced before undesirable process conditions are corrected. In papermaking, for instance, it is well known to employ on-line sensors that detect sheet properties such as basis weight, moisture content, and caliper.
It is well known that on-line measurements can be made by sensors that periodically traverse, or "scan", traveling sheets in the cross direction, i.e., in a direction extending across the sheet surface generally perpendicular to the direction of sheet travel. In practice, scanning sensors typically travel at a rate of about ten inches per second and provide an integrated output about every fifty milliseconds; thus, a typical scanning sensor provides about one measurement output per each half-inch in the cross direction. Measurement information detected by scanning sensors is often assembled to provide graphlike "profiles" composed of successions of measurements at adjacent locations, or "slices", in the cross direction. Cross-directional profiles also can be provided by measurement systems that employ arrays of fixed sensors that extend in the cross direction to simultaneously measure a sheet property at a plurality of slice locations.
To control sheetprocessing machines to produce uniform quality sheets, it is common practice to detect variations that occur both between and within cross-directional profiles. In the machine direction (i.e., the direction of sheet travel), sheet uniformity can be defined as the condition where the average value of a measured sheet property is constant from profile to profile; thus, machine-directional uniformity generally implies that cross-directional profiles do not change from scan to scan. In the cross direction, sheet uniformity can be defined as a measured sheet property being constant from slice to slice; thus, cross-directional sheet uniformity implies flat profiles.
Various devices are known to control sheet properties. In papermaking, for instance, a property known as "caliper" (i.e., sheet thickness) can be controlled by calendering and supercalendering machines In modern sheetmaking operations, a typical calendering or supercalendering machine comprises a series of rolls arranged in parallel, one above the other, in a "stack". The sheet material is then trained through the stack to pass through the nip areas between adjacent rolls. The effect of calendering and supercalendering is to change the caliper of sheet materials
In calendering and supercalendering machines, two of the basic parameters that can be continuously controlled are nip pressure and roll temperature. The two properties are interdependent, however, because an increase in temperature of a metal roll will cause it to physically expand, thereby increasing compressional pressure on sheet material at the nip area.
In practice, various actuators can control calendering pressure on calendering and supercalendering machines. For example, it is known to use edge loading devices that control the pressure loading on the ends of the rolls of a calender stack. Also, it is known to use crown rolls. Generally speaking, a crown roll contains an internal hydraulic mechanism which exerts hydraulic pressure to change the curvature laterally along a roll. In practice, the hydraulic mechanisms can either cause the lateral surface of a crown roll to have a convex, flat, or a concave shape. Also in practice, it is known to employ so-called sectionalized crown rolls. Those devices are divided lengthwise into zones, each of which can be controlled independently as a crown roll.
Also, the pressure exerted by calendering and supercalendering machines can be controlled by calender profile actuators. Those devices operate to heat calender rolls differentially along their lengths. In systems that employ calender profile actuators, the caliper of a sheet is detected at selected cross-directional locations following the calendering machine, and then selected sections, or "zones", of certain ones of the calender rolls are heated or cooled based upon the detected caliper.
One example of a system for controlling calender rolls is shown in U.S. Pat. No. 4,573,402. In the system described in the patent, sensors are provided to monitor hardness and uniformity of a reel during a reel-building process subsequent to calendering. Then, based upon the monitored values, actuators act upon the calender rolls to cause changes in their temperature and, hence, to change their diameters in a manner which adjusts paper hardness and uniformity. In particular, the patent describes a system wherein the actuators direct air through heating elements to reach selected sections of the calender rolls; also, the patent states that localized cooling of the calender rolls can be achieved by directing ambient air through ones of the heating elements that are not energized.
Another example of a calender control system is described in U.S. Pat. No. 4,114,528. In the system described in the patent, a paper web, after leaving a series of calender rolls, passes through a scanning device that generally continuously detects web-thickness information across the width of the web. According to the patent, the calender rolls are heated or cooled in response to detected differences in thickness of the paper web with the goal of achieving constant web thickness. More particularly, the patent states that air, either from a hot air plenum or from a cold air plenum, is selectively directed against individual longitudinal sections of the calender rolls in response to detected variations from normal thickness of the produced paper web.
Yet another example of a calender control system is set forth in U.S. Pat. No. 4,384,514. According to that patent, thermally-induced changes in nip clearance between calender rolls are achieved by devices that generate electromagnetic fields that interact with the metal of the rolls to provide induction heating at selected locations along the lengths of the rolls. The patent further suggest that sections of the rolls can be insulated from one another to localize the induction heating effects. In addition, the patent states that temperature increases at selected sections of the calender rolls can be controlled by auxiliary air or liquid cooling systems.
Also, U.S. Pat. No. 4,685,389 describes a calender profile actuator system. According to the patent, a plurality of nozzles are located at preselected locations along a plenum which extends the length of a calender roll. The nozzles each contain an individually controllable heating element for selectively heating the flow of air through the associated nozzles. The temperature of air ejected from the nozzles, in turn, controls the diameter of the calender roll and directly affects the thickness of calendered sheet material. A calender profile actuator system substantially as described in the patent has been commercialized by Measurex Corporation of Cupertino, Calif., and is known as the Caltrol control system. In that system, the caliper profile control actuators include devices which control high velocity air jets to impinge upon a calender roll over a temperature range from ambient (80.degree. F.) to 650.degree. F. In practice, the air flow from each jet is on half-slice centers.
In practice, calender profile actuator systems normally have relatively narrow control zones and relatively fast response times and, therefore, are well suited for correction of narrow (e.g. 0 to 300 mm.), short-term profile variations. However, actual caliper profiles often exhibit wider and more stable fluctuations, such as full-width slants, bowl shapes and s-curves. Conventional calender actuator profile systems are not well-suited for correcting such profile variations Instead, wide and stable fluctuations in caliper profiles can be better corrected by edge loading devices or by crown rolls of the sectionalized or non-sectionalized type.
A difficulty in using profile measurements for cross-directional control of sheet properties arises when a sheetprocessing machine, such as a calender stack, employs two or more profiling systems that are both capable of affecting the same property of sheet material. In such situations, the control issue is one of deciding how the various profiling systems should be controlled when the control signals, at any given time, are derived from the same cross-direction profile. For example, in the case of a calendering stack controlled by both a calender profile actuator system and a sectionalized crown roll, the control issue would be one of deciding how the control signals are to be distributed between the calender profile actuator system and the sectionalized crown roll.
In conventional control practice, multiple control systems on the same sheetmaking machines are often operated with actuating signals distributed, at any given time, according to a predetermined ratio, Such practice is often referred to as "ratioing" of signals. Another conventional strategy for operating two or more profile control systems on a sheetprocessing machines is called "mid-ranging". In a typical mid-ranging strategy, a first profile control system is operated as long as the set points of its actuators fall within predetermined limits, i.e., within a defined "mid-range". When one or more of the set points moves outside the preselected mid-range, the second profile control system is actuated Normally, the limits for the mid-ranges are selected based upon prior operating experience and upon the capabilities of the actuators in the control systems.
At this juncture, it should be appreciated that conventional control strategies, such as ratioing and mid-ranging, do not necessarily result in optimum use of the profile control system which is best suited to make adjustments to overcome detected non-uniformities in sheet profiles.
In U.S. patent applications Ser. No. 207,412, filed June 15, 1988, and commonly assigned herewith, the disclosure of which is herein incorporated by reference, there is disclosed a method and system for controlling two or more profiling systems which control the same property of sheet material which is being produced. The system described in the application employs control signals obtained from scanners that measure a given property of a sheet at numerous locations to provide cross-directional profiles. More specifically, the system described in the application employs a spatial-frequency splitting technique for separating, for each cross-directional profile, low spatial-frequency components from high spatial-frequency components. Then, the separated high and low spatial-frequency components are used to provide operating signals to control two or more profiling systems to modify properties of produced sheet material.